The lateralization of brain function (or hemispheric dominance or lateralization) is the tendency for some neural functions or cognitive processes to be specialized to one side of the brain or the other. The median longitudinal fissure separates the human brain into two distinct cerebral hemispheres connected by the corpus callosum. Both hemispheres exhibit brain asymmetries in both structure and neuronal network composition associated with specialized function. Lateralization of brain structures has been studied using both healthy and split-brain patients. However, there are numerous counterexamples to each generalization and each human's brain develops differently, leading to unique lateralization in individuals. This is different from specialization, as lateralization refers only to the function of one structure divided between two hemispheres. Specialization is much easier to observe as a trend, since it has a stronger anthropological history. The best example of an established lateralization is that of Broca's and Wernicke's areas, where both are often found exclusively on the left hemisphere in the vast majority of people. Function lateralization, such as semantics, intonation, accentuation, and prosody, has since been called into question and largely been found to have a neuronal basis in both hemispheres. Another example is that each hemisphere in the brain tends to represent one side of the body. In the cerebellum, this is the ipsilateral side, but in the forebrain this is predominantly the contralateral side.
Lateralized functions
Language and speech Language functions are lateralized to the left hemisphere in 96% of right-handers and 60% of left-handers.. The release of dopamine seems to regulate the activity in speech motor cortex and drive dominance in the left hemisphere. Meaning of words, called lexicon, is processed bilaterally which has been tested through the word superiority effect. This finding is consistent with the distributed memory and knowledge systems required for lexical entries; however, each hemisphere's lexicon is considered unique since it may be organized and accessed differently. For example, the right hemisphere lacks letter recognition, and cannot judge lexical relationships such as superordinate words or antonyms. The permitted organization of words, called grammar, is lateralized in only one hemisphere, typically the left one. These functions include "understanding verbs, pluralizations, the possessive, and active-passive differences" and understanding changes in meaning due to word order. However, the right hemisphere is able to judge when a sentence is grammatically correct, which may indicate that patterns of speech are learned by rote rather than applied through understanding rules. Speech production and language comprehension are specialized in Broca's and Wernicke's areas respectively, which are located in the left hemisphere for 96% of right-handers and 70% of left-handers. However, there are some cases in which speech is produced in both hemispheres in split-brain patients; lateralization can also shift due to plasticity over time. The emotional content of language, called emotional prosody, is right-lateralized. In writing, studies attempting to isolate the linguistic component of written language in terms of brain lateralization could not provide enough evidence of a difference in the relative activation of the brain hemispheres between left-handed and right-handed adults.
Sensory processing Sensory processing for the left and right sides of the body is often lateralized to the contralateral hemisphere due to nerve fiber decussation. Because of the functional division of the left and right sides of the body, the processing of information in the sensory cortices is essentially identical. That is, the processing of visual and auditory stimuli, spatial manipulation, facial perception, and artistic ability are represented bilaterally. Numerical estimation, comparison and online calculation depend on bilateral parietal regions while exact calculation and fact retrieval are associated with left parietal regions, perhaps due to their ties to linguistic processing.
Vision
In vision, retinal ganglion cells undergo partial decussation at the optic chiasm, where axons from the nasal retinas cross to the opposite hemisphere, while axons from the temporal retinas remain on the ipsilateral side. As a result, visual input from the left visual hemifields are processed by the right hemisphere's visual cortex, while input from the right visual hemifields are processed by the left hemisphere's visual cortex.
Hearing In hearing, spiral ganglion neurons in the vestibulocochlear nerve project to the ipsilateral cochlear nuclei in the medulla. However, second-order axons from the ventral cochlear nucleus branch to both the ipsilateral and contralateral superior olivary complexes. Consequently, hearing is strongly lateralized only at the ipsilateral cochlear nuclei, while further processing in the inferior colliculi, the medial geniculate nucleus of the thalamus, and the auditory cortex occurs bilaterally with a slight contralateral dominance. This lateralization explains why damage to one cochlear nucleus causes deafness in the ipsilateral ear, whereas damage above the cochlear nucleus typically results in only slight hearing loss. When tasked to repeat words in a dichotic listening task, individuals tend to say words played in their right ear, a phenomenon called right-ear advantage. Since hearing is slightly contralateral dominant, this effect is consistent with the left hemisphere lateralization of language. When tasked to recall melodies in a dichotic listening task, people instead tend to have a left-ear advantage.
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